1. Field of the Invention
This invention relates to high-pressure sodium vapor lamps of the kind wherein arc discharge occurs in a vapor of sodium and mercury at a sodium vapor pressure of tens of Torr, and particularly to the composition of the amalgam which produces the requisite vapor for lamp operation.
2. Description of Related Art
The operating characteristics of sodium vapor electric discharge lamps are largely determined by the composition and pressure of the vapor as well as of the rare gas, such as neon, argon, xenon or mixtures thereof, which is included to initiate the arc discharge. A low pressure sodium lamp typically contains sodium vapor at a partial pressure of a few milli-Torr as well as starting gas at a pressure of about 20 Torr, and provides high luminous efficiency in the monochromatic yellow spectral region. Much broader spectral luminosity is achieved by the high-pressure sodium lamp, which contains mercury as well as sodium vapor in a sodium-to-mercury atomic ratio of 2 or 3:1. The requisite vapor is established by charging such lamps with sodium amalgam, the vapor pressure characteristics of which result in lamp operation at a mercury partial pressure of about one atmosphere (760 Torr) and a sodium partial pressure of at least 60 Torr, the latter usually not exceeding 80 Torr. However, the sodium radiation covers a broad band of color and exceeds the power radiated by the mercury in its characteristic ultraviolet spectral region. The mercury vapor increases the operating voltage of the lamp and reduces the current, thereby improving operating efficiency.
The operating life of a high-pressure sodium vapor ("HPS") lamp is an important reason for its commercial success, the rated life of a 400 watt HPS lamp being about 22,000 hours. A significant factor limiting the life is that the lamp operating voltage increases as the lamp is continued in service. This is due in large part to sputtering of the surface of the electrodes each time the lamp is turned on. Such sputtering results in the transport of electrode material, such as tungsten and the electron emissive coatings thereon, to the walls of the arc tube and causes blackening of the arc tube end-chamber. This raises the temperature of the tube, increasing the vapor pressure of the mercury and sodium therein. Applicants have found that the sputtering phenomenon is dependent on the time required for the lamp to reach its steady-state operating voltage after being turned on, and that more rapid attainment of the steady-state condition will result in decreased sputtering and therefore in increased lamp life.
It is known that the inclusion of various auxiliary metals in an electric discharge lamp can produce significant changes in the lamp operating characteristics. For example, U.S. Pat. No. 3,629,641, issued Dec. 21, 1971, discloses a low-pressure mercury vapor discharge lamp, e.g., a fluorescent lamp, in which the luminous efficiency is rendered less temperature dependent by incorporating indium or indium amalgam therein in an indium-to-mercury ratio of from 3:1 to 12:1 by weight. U.S. Pat. No. 3,678,315 issued July 18, 1972, discloses a low-pressure sodium vapor lamp in which the inclusion of indium in an atomic concentration exceeding that of the sodium reduces the temperature dependence of the sodium vapor pressure during lamp operation, thereby maintaining high luminous efficiency even when operating at high lamp current levels. However, the problem of electrode sputtering during start-up of a high-pressure sodium vapor lamp has not heretofore been resolved.